Currently, both car phones and hand-held phones are used in radio phone networks. The greatest difference therebetween is the higher output power of the car phone and the greater requirements as regards sensitivity and error repetitions. In the present patent application the characteristic features of the car phone also refer to the properties of a combination of a hand-held phone and an amplifier i.e. booster.
In the prior art designs a booster comprises a synthesizer, a processor and radio unit of its own, in addition to the standard RF components. One of the state of the art designs is the so-called serial number transmission system in which the booster is a complete car phone without a fixed serial number of its own, i.e. a so-called NAM booster (Number Assignment Module). When a phone is connected to a booster, the NAM data, comprising the phone number of the phone, is transmitted electrically to the booster which thereafter serves ms a phone, and the hand-held phone serves thereafter merely as an operating means, in other words, no RF connection between the hand-held phone and the booster exists.
In another conventional design, the booster comprises two duplex filters (one for the phone terminal and the other for the antenna terminal), and rf components therebetween. An advantage of the design is a simple RF connection, comprising only one coaxial cable between the booster and the telephone. A drawback of the design is that higher performance values are required of the hand-held phone than the specifications set for the radio phone system would usually require of it, so that because of the booster, the hand-held phone has to be produced with unnecessary high performance, resulting in it being expensive to produce and manufacture.
The prior art technology is described below referring to the accompanying FIG. 1, showing the principle of how to combine a hand-held phone and an amplifying means known in the art.
FIG. 1 presents a combination of a hand-held phone and an amplifying means known in the art. A connection point for the amplifying means 1 and the hand-held phone 2 is an antenna terminal 3 of the hand-held phone 2. Instead of a standard antenna, the hand-held phone 2 is connected to an amplifying means 1, that is, to a booster 1, which is usually also connected via a duplex filter 6' to an antenna ANT. A duplex filter 6 is placed between the receiver branch RX.sub.v, the preamplifier 4 of the amplifying means 1 and the power surge 5 of the transmission branch TX.sub.v. A duplex filter 7 of similar type is provided in the hand-held phone 2. The duplex filters 6 and 7 separate the transmission and reception frequencies within the amplifier into their equivalent branches, these being respectively amplified.
In the state of the art there is a difficulty in implementing the properties required of the hand-held phone. Owing to the attenuation caused by the duplex filters of the booster, the signal has to be amplified in the booster with an amplifier 4 in order to meet the sensitivity requirements. Now, the signal processing properties of the combination provided by the booster 1 and the hand-held phone 2, such as spurious response properties (high signal handling, intermodulation etc), are deteriorated, said properties being mainly limited by the high frequency parts of the hand-held phone. The problem has to some extent been ameliorated by increasing the supply current of the front end stage (not shown) of the hand-held phone. However, the increased current consumption in using the hand-held phone leads to shortened life time of the hand-held phone or to the use of more powerful batteries, this being entirely contrary to the well known objectives of increased battery life and/or low volume and weight of hand-held phones. In addition, since a reception signal amplified already in an external amplifier is conducted to the front end amplifier of the radio phone, intermodulation effects in the signal are exacerbated.